This invention relates generally to a combustion system and method utilizing a furnace in which a mixture of air and fuel, such as coal, is discharged from one or more burners in a tangential direction with respect to an imaginary circle in the center of the furnace and, more particularly, to such a system and method in which secondary air is also discharged into the interior of the furnace in a combustion-supporting relationship to the fuel.
In coal-fired combustion systems, a mixture of coal and primary air is usually discharged from one or more burners mounted relative to a furnace wall or walls, while secondary air is discharged from one or more air nozzles located adjacent each burner. Many types, arrangements and locations of the burners and the secondary air nozzles have been used. For example, in a conventional, straight-firing system, the burners and the secondary air nozzles are mounted relative to the furnace walls in a manner to respectively discharge the coal/primary air mixture and the secondary air in a direction perpendicular to the walls. In contrast, and in an effort to improve the combustion efficiency among other things, a tangential firing system has evolved in which the burners and the air nozzles are disposed in each of the corners of the furnace. In these tangential arrangements, the burners and secondary air nozzles are located and designed to respectively discharge the coal/primary air mixture and the secondary air in a direction generally tangentially to an imaginary circle in the center of the furnace. In some of these designs the burners discharge the coal/primary air mixture tangentially with respect to one circle, and the air nozzles discharge the secondary air with respect to another circle having a different diameter. The burners and the air nozzles are also often tiltable about a horizontal axis to enable their air discharge pattern to be varied in a vertical direction, i.e., along the height of the furnace which, among other things, enables the furnace temperature to be controlled.
However, since all of the secondary air is discharged towards the center of the furnace interior, a reducing atmosphere is often present along the inner surfaces of the boundary wall which causes corrosion and slagging.
Also, since these types of furnaces are usually asymmetrical in plan view, that is, two opposed furnace walls are relatively short and the other two opposed walls are relatively long, each air nozzle must be designed with a specific discharge pattern depending on the particular corner of the furnace in which it is mounted and the particular size of the imaginary circle to which its discharge pattern is directed. This, of course, adds to the cost of the system.
Therefore, what is needed is a tangentially fired combustion system and method in which corrosion and slagging along the inner surfaces of the furnace boundary walls are minimized.